The two most common processes to extract edible oil from oleaginous vegetable materials such as seeds, beans, and nuts are the full press extraction process where mechanical screw presses are used to extract the oil by squeezing the oleaginous vegetable material under high pressure, and the solvent extraction process where a solvent is mixed with the oleaginous vegetable material to dissolve and extract its oil content. Solvent extraction inevitably requires a careful preparation of the oleaginous vegetable material comprising for example de-hulling, heating, flaking, and cooking. The solvent is usually hexane.
Full press extraction is found in small oil mills, but this mechanical process extracts only about 90% of the oil contained in the oleaginous vegetable material, which is unacceptable for larger oil mills processing commodities.
For oleaginous vegetable material containing a large proportion of oil, for example about 40% in weight, such as rapeseed or sunflower seeds, it is advantageous to realise the oil extraction in two steps: a partial mechanical extraction using a prepress followed by a solvent extraction using an extractor. As a matter of fact, the mechanical extraction can be seen as a preparation or a step of the preparation during which a significant fraction of the oil is extracted with a controlled mechanical process. For rapeseed and sunflower seed for example, it has been found advantageous to squeeze about two third of the oil (66%) mechanically and to extract the remaining oil by solvent extraction.
Accordingly, for such oleaginous vegetable material containing a large proportion of oil, the first oil extraction step is a mechanical extraction using a prepress that will produce a prepress-oil and a prepress-cake that from now will be designated as “cake”. The cake still contains a substantial amount of oil, typically about 15-25% in weight. The second step is the solvent extraction of the cake, whereby a meal is produced that is substantially free of oil (ideally below 0.5% in weight) and a miscella rich in oil. During the prepressing step, the vegetable oleaginous material is subjected to considerable friction and compression forces generating significant heat surge. Consequently, the cake exits the prepress typically at a temperature of 105° C. to 115° C. To be complete, upon exit of the prepress, a quick cooling due to flash evaporation of a fraction of the moisture naturally present in the oleaginous material takes place and brings the cake temperature down to about 100° C.
In some oil mills, the hot cake is introduced in the solvent extractor without methodical and controlled cooling, and therefore its temperature exceeds the boiling point of the solvent (usually hexane) used in the solvent extractor. This is not a safety issue as long as the extractor is equipped with a properly sized condenser that will condense any evaporated solvent.
However, the real issue is the fragility of the cake when this one is hot. As a matter of fact, the prepressing should be an excellent preparation to the solvent extraction since most of the cells containing the oil are ruptured by the considerable friction and compression forces generated in the prepress. Furthermore, as previously mentioned, at the exit of the prepress, the water flashes, creating a porous structure that should be ideal for solvent diffusion inside the cake. However, this ideal structure is ruined because the cake must be transported hot over a long distance from the prepress to the extractor. Unfortunately, when hot, this one is very fragile and is broken into small pieces and fines (dust) during the long transport from the prepress to the extractor. Indeed, typically, the extractor is located in a dedicated building where specific safety rules do apply in reason of the explosive risk generated by the hexane. Therefore, typically, an oil mill facility will include several prepresses located in the preparation building and one extractor located in a dedicated and restricted access building. Accordingly, due to the long transport of the fragile hot cake, a lot of fines are generated and consequently poor percolation is observed which is at the root of suboptimal solvent extraction performances of the extractor, such as long residence time and/or high residual oil and/or decreased capacity. Indeed, in the field of solvent extraction, it is well known that the occurrence of fines (in the material to be solvent extracted) have a negative impact on solvent percolation which is of outmost importance for the performance of a percolation solvent extractor.
Therefore, to remedy to the situation described above, in most oil mills, the cake is methodically cooled at about 60-80° C. in a horizontal cooling tunnel before being introduced in the solvent extractor. The cake is introduced through a feeding hopper and while being slowly transported through the horizontal cooling tunnel on a moving belt for example, it is slowly cooled by a stream of cross flow air which is then released to the atmosphere. The cooled cake is then conveyed to the solvent extractor.
Nevertheless, such cake cooling as currently realized in a horizontal cooling tunnel is unsatisfactory. Indeed, before the actual cooling of the cake is realized, said cake must be transported from the prepress to the horizontal cooling tunnel. As a matter of fact, oil mills are equipped with several prepresses and only one horizontal cooling tunnel, and therefore, the transport of the cake will require several conveyors and will be relatively long and thus the cake will already have the time to be broken down into small pieces and fines before being cooled and stiffened. Consequently, even if a limited improvement of the percolation rate is observed, the solvent extractor performance remains suboptimal in terms of residence time and/or capacity and/or residual oil remaining in the extracted vegetable material. The suboptimal performance of the extractor originate from the low percolation rates induced by too many fines. Fines are created by too much conveyance of the hot, fragile cake before this one is cooled and stiffened.
Another issue associated to the horizontal cooling tunnel currently practised in the field is that the air used in the air cooler is released to the atmosphere and pollutes the surroundings with malodourous components especially when processing rapeseed. It is technically possible to treat the air to remove most of the malodorous components with for example a bio-filter, but this option is particularly expensive because the air volume to be treated is very large. Typically for a horizontal cooling tunnel currently available, 300 m3 of air is needed per ton of seed (or 425 m3 of air per ton of cake). Therefore, for an oil mill having a capacity 750 ton per day, it means that 225,000 m3 are released daily to the surroundings or must be treated a high cost.
Therefore, there is a need in the art for an improved equipment and process for the transport of the cake from the mechanical prepress to the subsequent solvent extraction, that permits to improve the subsequent solvent extraction performance and lessen the release of malodorous components into the surroundings.